First of all, you only need to run migrate.py with option --run-migrations=8.0 in order to migrate from 7.0 to 8.0

You need to fix all ERRORS messages (INFO and WARNINGS could be ignored firstly).

After the migration process, try to update all the database (-u option) on V8 source code before going to production. You can also use the OCA's module Database cleanup to clean your database from obsolete data (use this cautiously).

updating the xx_migrated database -u option? could you add a bit on how to do that please ?